The present invention is generally related to phonophoresis and is more particularly directed to the intravascular generation of ultrasonic energy for enhanced delivery of drugs and also for the dual purpose of intravascular blockage removal and the prevention of restenosis.
It is known that ultrasound can transport a physiologically active agent, or drug, across a membrane. Phonophoresis is defined as the migration of drug molecules, contained in coupling/contact agents, through the skin under the influence of ultrasound. It has been demonstrated that ultrasound can drive a drug into skeletal muscle and paravertebral nerve, and an increase in percutaneous absorption of several drugs by the influence of ultrasound has been reported by many observers. The usefulness of phonophoresis in chemotherapy is important since the ultrasound localizes drug delivery to a desired area, thus increasing the effectiveness without increasing systemic toxicity.
Ultrasonic energy has been considered for removal of intravascular blockages due to both atherosclerotic plaque and intravascular blood clots. It has been shown that ultrasonic energy is useful in fragmenting plaque and thrombosis, either as a result of mechanical action thereon or cavitation thereof, in which high energy ultrasound applied to liquids generates vapor-filled microbubbles, or "cavities," with the concomitant rapid expansion and collapse of the cavities, accompanied by local intense hydraulic shock leading to fragmentation or dissolution of the thrombosis.
As set forth in co-pending application Ser. No. 07/625,919 filed Dec. 10, 1990, a miniature ultrasonic ablation tool mounted at the tip of a flexible catheter and deployed in small diameter arteries is useful for removal of intra arterial atherosclerotic plaque and/or blood clots. This reference is incorporated herewith, including all drawings and specification, by this specific reference thereto.
While transluminal coronary angioplasty has gained wide acceptance, acute reocclusion after successful coronary angioplasty occurs in approximately 5% of patients. Late restenosis (generally in less than three months) is a major clinical problem limiting the long-term efficacy of this treatment of atherosclerosis and occurs in 25% to 50% of patients.
The pathogenesis of restenosis is still not entirely clear, but angloscopic observations, autopsy specimens from patients undergoing coronary angioplasty recently, and atherectomy samples from patients with restenosis have demonstrated that the pathophysiologic process leading to acute occlusion after coronary angioplasty are mainly related either to predominantly thrombotic mechanisms or major plaque dissection with intimal flaps and superimposed thrombosis, and that the pathophysiologic events leading to chronic restenosis involved vascular injury, platelet deposition and thrombosis, organization and incorporation of thrombi and subsequent smooth muscle cell proliferation and connective tissue synthesis.
Because excessive platelet deposition at the site of coronary angioplasty and finally smooth muscle cell proliferation appears central to the process of restenosis, the use of an agent directed at the thrombus and smooth muscle cell has been considered. A number of agents have been reported to inhibit smooth muscle cell proliferation, including heparin, immunosuppressants such as cyclosporin, fish oil and omega-3 fatty acid, calcium channel blockers, corticosteroids, captopril and other inhibitors.
The ultrasonic apparatus of the present invention is specifically useful for the enhanced phonophoresis and the dissolution of intravascular blockages with concomitant treatment for inhibition of restenosis.